Specify where Additional Boards Manager URLs is (#7733)

As a newcomer to Arduino IDE, it is not obvious where the "Additional Boards Manager URLs" field is

README.md

@@ -30,7 +30,7 @@ Starting with 1.6.4, Arduino allows installation of third-party platform package
 
 - Install the current upstream Arduino IDE at the 1.8.9 level or later. The current version is on the [Arduino website](https://www.arduino.cc/en/main/software).
 - Start Arduino and open the Preferences window.
-- Enter ```https://arduino.esp8266.com/stable/package_esp8266com_index.json``` into the *Additional Board Manager URLs* field. You can add multiple URLs, separating them with commas.
+- Enter ```https://arduino.esp8266.com/stable/package_esp8266com_index.json``` into the *File>Preferences>Additional Boards Manager URLs* field of the Arduino IDE. You can add multiple URLs, separating them with commas.
 - Open Boards Manager from Tools > Board menu and install *esp8266* platform (and don't forget to select your ESP8266 board from Tools > Board menu after installation).
 
 #### Latest release [![Latest release](https://img.shields.io/github/release/esp8266/Arduino.svg)](https://github.com/esp8266/Arduino/releases/latest/)